Dracut

From FedoraProject

Dracut is an initramfs infrastructure. Information about the project's goals and aims can be found at Initrdrewrite. Unlike previous initramfs systems, Dracut aims to have as little as possible hard-coded into the initramfs.

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Technical details

With Dracut, the initramfs has one purpose in life - getting the rootfs mounted so that we can transition to the real rootfs. This is all driven by device availability. Therefore, instead of scripts hard-coded to do various things, we depend on udev to create device nodes for us and then when we have the rootfs's device node, we mount and carry on. This helps to keep the time required in the initramfs as short as possible so that things like a five second boot aren't made impossible as a result of the very existence of an initramfs. It's likely that we'll grow some hooks for running arbitrary commands in the flow of the script, but it's worth trying to resist the urge as much as we can as hooks are guaranteed to be the path to slow-down.

Most of the initrd generation functionality in dracut is provided by a bunch of generator modules that are sourced by the main dracut script to install specific functionality into the initrd. They live in the modules subdirectory, and use functionality provided by dracut-functions to do their work. Some general rules for writing modules:

Use one of the inst family of functions to actually install files on to the initrd. They handle mangling the pathnames and (for binaries, scripts, and kernel modules) installing dependencies as appropriate so you do not have to.

Scripts that end up on the initrd should be POSIX compliant. Dracut will try to use /bin/dash as /bin/sh for the initrd if it is available, so you should install it on your system -- dash aims for strict POSIX compliance to the extent possible.

Hooks MUST be POSIX compliant -- they are sourced by the init script, and having a bashism break your user's ability to boot really sucks.

Generator modules should have a two digit numeric prefix -- they run in ascending sort order. Anything in the 90-99 range is stuff that dracut relies on, so try not to break those hooks.

Generator modules and hooks must have a .sh extension.

We have some breakpoints for debugging your hooks. If you pass 'break' as a kernel parameter, the initramfs will drop to a shell just before switching to a new root. You can pass 'break=hookpoint', and the initramfs will break just before hooks in that hookpoint run.

Also, there is an attempt to keep things as distribution-agnostic as possible. Every distribution has their own tool here and it's not something which is really interesting to have separate across them. So contributions to help decrease the distro dependencies are welcome.

Using Dracut

Right now, most of the testing is done using a qemu/kvm guest and generating the initramfs on another box but the support is all present to build for the "running" machine. For the former, you can boot the guest using qemu's -kernel and -initrd options.

The command dracut exists and will build an image. It is command-line equivalent to most mkinitrd implementations and should be pretty straightforward to use.

To use, just run dracut with an output file name and, optionally, a kernel version (it defaults to using the current). The appropriate modules will be copied over and things should be good to go. If you'd like to customize the list of modules copied in, edit /etc/dracut.conf and set

dracutmodules="foo bar baz"

Dracut calls functional components in modules.d modules while kernel modules are called drivers.

Try to boot from that image by modifying /etc/grub.conf. Be sure to have a fallback entry.

Additional Dracut kernel command line parameters can be found on the options page and in the man page.

When you have become familiar with Dracut take some time to look at the Test Matrix and see if you can contribute to testing.

Getting the Source

The primary methods of distributing the Dracut source are source RPMs in the Fedora development tree and git. To access the current source code in in non-rpm format, you'll need to install git: yum install git.

Note that several related packages will be installed as well. After the git source code management tool has been installed, then you use anonymous git access to the Dracut repository.

git clone git://dracut.git.sourceforge.net/gitroot/dracut #FIXME

If you have committer access to Dracut, then you will want to use the git+ssh access URL.

git clone git+ssh://dracut.git.sourceforge.net/gitroot/dracut #FIXME

Once you've committed changes locally, you can push them with git push.

If you would just like to browse the Dracut git repository via the web, then you can use Dracut gitweb.

Reporting Problems

Before filing a bug, please read up on debugging, which will tell you how to fill out useful bug reports that will help us quickly solve your problem. Also take a look at Dracut or try searching Bugzillafor other reports about your problem, as some bugs are often filed by several people.

Dracut Team

In alphabetical order, the following people are the Dracut team and are responsible for the majority of commits.